1. Field of the Invention
The present invention relates to the preparation of polyisocyanates containing a biuret radical, and, more especially, to the preparation of biuret polyisocyanates by reacting an aliphatic, alicyclic or arylaliphatic diisocyanate with a biuret-forming reactant.
2. Description of the Prior Art
Polyisocyanates containing a biuret group have known utility in the production of foams, adhesives and paints.
Among these applications, the use of such polyisocyanates as constituents of paints is becoming increasingly important, especially in the automotive industry.
Paint films produced from aromatic polyisocyanates are also known to turn yellow and crack.
In contrast, paint films prepared from aliphatic, alicyclic or arylaliphatic polyisocyanates retain their properties for very long periods of time and are therefore particularly suitable in the motor vehicle field.
However, the preparation of aliphatic, alicyclic and arylaliphatic polyisocyanates, using diisocyanates and water, presents certain disadvantages.
Thus, according to U.S. Pat. Nos. 3,124,605 and 3,903,127, the reaction of the diisocyanate monomer with water produces polyurea, which precipitates. The reaction between the diisocyanate monomer and water (which is dissolved in small quantities in said diisocyanate) produces a polyisocyanate containing a biuret group. But the reaction of water with small amounts of the diisocyanate monomer that can dissolve therein, results in the production of a polyurea.
As a solution to this problem, the preparation of these polyisocyanates by reacting the diisocyanate monomer with water, at a temperature of at least 70.degree. C. and in a mixture of an ethylene glycol derivative, such as an ethylene glycol methyl ether acetate, and a methyl or ethyl phosphate, has been proposed. See published French Patent application No. 2,382,468.
This process enables the precipitation of polyurea in polyisocyanate to be practically avoided.
However, it also transpires that, when solutions of polyisocyanates containing a biuret group thus prepared are stored for a few days, their viscosity increases, polybiurets are formed, and the free diisocyanate monomer content increases.